Although colored lead pencils have provided enjoyment to children and others for a long time, attempts have been made to improve upon the properties of these pencils. The properties that make coloring a pleasant and rewarding experience include smooth laydown of the lead composition, good color intensity of the markings, good break strength, and sufficient lead bending strength. In addition, it is desirable to have colored lead pencils that can produce markings on paper that do not smear easily and that are erasable by ordinary pencil erasers.
Common colored pencil leads are made from a blend of a pigment, a binder resin and a filler. Although these colored leads possess excellent writing qualities, their markings cannot be easily erased by ordinary pencil eraser. Attempts have been made to improve the lead properties, particularly the erasability of the markings. For example, attempts have been made to improve erasability by reducing the pigment content of pencil leads; although this approach reduces the color contrast between the erased mark and the paper, the color intensity of the original mark has been unsatisfactory.
In another approach to improve erasability of the markings, colored pencil leads have been made by blending a mixture of a pigment, one or more fillers, and a wax. Although this approach has improved the erasability of the marking, complete removal of the marking has not been achieved with the result that the erased region is left with a smeared appearance. It has been found that the wax tends to adhere rather tenaciously to the fibers of the paper and to penetrate the spaces between the paper fibers, and when the markings are rubbed, they tend to smudge, smear, and/or leave a residue.
Attempts have been made to prepare colored lead pencils to eliminate the smearing or residue. For example, the amount of wax has been reduced. Unfortunately, this approach makes the lead very hard and the lead does not write well.
In yet another approach, the use of a low melting wax, fat, or oil has been attempted. Colored pencil leads have been made by blending a pigment, a binder resin, a filler, and a low melting wax, fat, or oil. This approach, although it improves the erasability of the markings, is not easily practicable in commerce because either it increases the manufacturing cost or it affects certain other qualities of the lead. For example, if the lead is produced in a wet lead forming process, the wet lead should be dried at a temperature that is lower than the melting temperature of the wax, fat or oil. This increases the lead dry time, and consequently, reduces manufacturing productivity. The low melting wax, fat or oil also tends to seep or migrate out of the lead. This could lead to loss of strength as well as failure to produce an appropriate color strength. Further, the low melting wax, fat or oil also tends to bleed through the paper, thereby further affecting the quality of the coloring. To alleviate some of the drawbacks associated with the use of a low melting wax, fat, or oil, complex manufacturing processes have been proposed which unfortunately can increase the pencil manufacturing cost. Thus, there exists a need for a colored pencil lead having improved erasability. There further exists a need for colored pencil leads which do not smear when erased by an ordinary pencil eraser. There further exists a need for colored pencil leads which do not smudge when rubbed with a finger, blending stump, or other objects. There further exists a need for a colored pencil lead that has smooth laydown, good color strength, adequate bending strength, and adequate break strength.
These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.